This invention pertains to illumination apparatuses that are suitable for photo optical systems, particularly for projecting and exposing semiconductor or liquid-crystal patterns, which are formed onto masks, onto photosensitive substrates. This invention further pertains to projection and exposure apparatuses and exposing methods that use said illumination apparatuses.
In the recent years, projection and exposure apparatuses that use a KrF excimer laser as a light source, particularly semiconductor manufacturing projection and exposure apparatuses (KrF excimer stepper) have been produced. FIG. 6 is a schematic diagram of a projection and exposure apparatus with prior art illumination apparatus equipped. After a beam generated from a KrF excimer laser 100 has been expanded by using a beam expander 102, said expanded light beam is reflected with a vibration mirror 104, and said reflected light enters a fly eye lens 106. The beam whose wave front is divided by using the fly eye lens 106 illuminates a reticle 110 via a condenser lens 108. A circuit pattern displayed onto reticle 110 is transferred onto a wafer 114 by using a projecting lens 112. In this case, a single exposure is performed by several tens of pulse radiations. The fly eye lens 106 is a necessary element for correction of an nonuniform intensity of the Gaussian distribution which the laser beam has. However, the beam whose wave front is divided by the fly eye lens 106, is overlapped onto the reticle again, an interference noise is generated onto the reticle. As for a first prior art projection and exposure apparatus shown in FIG. 6, the angle of vibration mirror 104 is adjusted per pulse radiation by using a mechanism as not shown in the drawing, the interference noise is averaged and from this, the distribution of the illumination intensity on the reticle is made to be uniform.
FIG. 7 is a schematic diagram of an illumination apparatus disclosed in Japanese unexamined patent application No. S63-216338, which is a second projection and exposure apparatus. A light beam radiated from an excimer laser 200 is converted into a group of beams parallel to the line direction, which are as the same number as that of element lenses of a fly eye lens 204, by using a multi-reflection mirror system (a multi-beam optical system) 202 that comprises a total reflection mirror 202C and a partial reflection mirror 202R. Each beam enters each element lens of fly eye lens 204. A beam radiated from a secondary light source 206 that is formed corresponding to each element lens is radiated onto a mask (a reticle) 210 by a condenser lens 208. As for said prior art projection and exposure apparatus, by adjusting the distance between total reflection mirror 202C and partial reflection mirror 202R, the difference in length of the beam passage of each beam is determined so as to be the distance that can be interfered by excimer laser 200 or longer. When the difference is determined as described above, beam waves generated from the fly eye lens 204 do not interfere with each other; as a result, an interference noise is not generated onto a mask (a reticle).
FIG. 8 is a diagram illustrating a third illumination apparatus and a projection and exposure apparatus that equips said illumination apparatus as disclosed in Japanese unexamined patent application No. H10-125585. A two-dimensional multi-beam forming optical system 308 that comprises a first one-dimensional multi-beam forming optical system 310 and a second one-dimensional multi-beam forming optical system 312 forms a group of Nxc3x97M numbers of two-dimensional beams from a beam generated from a laser light source 300. The group of said Nxc3x97M numbers of said two-dimensional beams enter a fly eye lens 320a, a condenser lens 322a, a fly eye lens 320b, a condenser lens 322b, and a reticle 328 in that order. The first multi-beam optical system 310 and the second multi-beam optical system 312 are orthogonally arranged; the difference in length of the light passages of the first and second multi-beam optical systems is optimized such that all the beams do not interfere with each other. The reflection ratio of each section of the partial reflection mirror that is a component of the first and the second multi-beam optical systems is also optimized such that each intensity of two-dimensional beam arrays that are subsequently generated. Each beam generated from the multi-beam optical system is made to enter the fly eye lens 320a while it is expanded to an effective diameter of the element lens of the fly eye lens or larger by using diffusion plates 314a and 314b. 
As for excimer lasers which are light sources for projection and exposure apparatuses, the width of the wave length has been reduced. For said reason, in addition to the time coherence, the spatial coherence of recent excimer lasers has also increased in comparison with that of conventional excimer lasers. As the spatial coherence increases, the contrast of interference noises by using the fly eye lenses increases. Said interference noises cause pattern transferring errors when they are superimposed onto circuit patterns. More specifically, ununiform exposure components that have a fine structure increase. When an exposure apparatus is structured with the first prior art illumination apparatus as described above, using a KrF excimer laser having a narrower width of wave length and when an exposure is made with several tens of pulses, the Gaussian intensity distribution of the laser beam can be averaged; however, fine interference noises cannot be sufficiently averaged, which is a disadvantage of the prior art. When the number of exposure pulses (an average number) is increased while reducing the exposure intensity, the interference noises are reduced; however, the throughput also decreases.
The second prior art is a method to reduce the effect of a spatial coherence without increasing the number of exposure pulses. The second prior art method aims to obtain an effect equivalent to the increase of exposure pulses, by converting a beam from a light source into multiple beams that do not interfere with each other.
However, when a projection and exposure apparatus is provided by using the illumination apparatus, it is necessary to increase the number of multiple beams to 50 or more in order to reduce the effect of an interference noise when a single reflection layer is used. For said reason, the size of a multi-beam optical system increases in the reflecting direction of the beams. Additionally, because each beam is projected to an element lens of a fly eye lens without expanding it, the nonuniformity of the intensity with the Gaussian distribution is presented, which is specific to laser beams. It is also difficult to project a beam having a uniform shape to each element lens of a fly eye lens. Therefore, when the illumination apparatus is used, an illumination with a practical uniformity cannot be obtained as similarly to the other case as described above; the uniformity of intensity on a reticle cannot be sufficiently improved; as a result, a pattern transfer error occurs. More specifically, as for the embodiments shown in FIGS. 6 and 7, either of the disadvantages, such as the nonuniformity of the Gaussian intensity distribution or the interference noise generated by a fly eye lens, can be solved; however, but not both at the same time.
The third prior art illumination apparatus is compact and does not reduce the throughput; said illumination apparatus also has a structure such that both ununiformity of the Gaussian intensity distribution and interference noise of a fly eye lens can be reduced at the same time. As for the third prior art illumination apparatus, in order to reduce ununiformity of the Gaussian intensity distribution, the effective diameter of each beam generated is expanded to the effective diameter of an element lens of the fly eye lens or larger. In order to generate multiple compact incoherent beams, two one-dimensional multi-beam optical systems are arranged orthogonally; the differences in length of the beam passages generated in the first multi-beam optical system and the second multi-beam optical system are adjusted to different values.
However, as in the third prior art illumination apparatus, it is necessary to align a beam which is sequentially generated by a repetitive reflection within a predetermined effective diameter of each partial reflection mirror; it is difficult to adjust said beam. Further, when a multi-beam optical system is formed, it is necessary to have a process that vapor-deposits multiple types of partial reflection mirrors with various transmissivities (or reflection rates) onto a single substrate; for said reason, the manufacturing cost increases. As stated in the foregoing, according to the third prior art illumination apparatus, it is difficult to attain a low interference noise, a lower cost, and a high throughput at the same time.
Next, problems pertaining to the increase in size of the second prior art illumination apparatus is further discussed in detail. FIG. 9 is a diagram of a one-dimensional multi-beam optical system as in the second prior art example. In this case, the direction of an incident beam is indicated by an axis z; the direction of a repetitive reflection is indicated by an axis y; the direction perpendicular to the sheet surface is indicated by an axis x. As for said structure shown in FIG. 9, total reflection mirror 202C with an r0 energy reflection rate and a partial reflection mirror 202R with an r energy reflection rate are arranged in parallel facing each other. While an incident beam T0 partially transmits mirror 202R, it repeatedly reflects between two mirrors; by means of this, incident beam T0 is converted into multiple beams parallel to each other. In this case, when the difference in length of the beam passage of each beam is adjusted to a time coherence distance of the light source or more, the interference noise can be reduced.
In FIG. 9, an installing angle xcex8 in relation to incident beam T0 and a vertical distance d of two mirrors are indicated by the following formulas:
xcex8=tanxe2x88x921(h/L)xe2x80x83xe2x80x83(1)
d=h/(2 sin xcex8)xe2x80x83xe2x80x83(2)
In this case, L stands for a difference in length of light passages between ABC and AD; h stands for a vertical distance of each beam. The difference in length L is set equal to or greater than a time coherence distance Lc of the light source in order to reduce the interference noise. Due to a geometric limitation, it is necessary to adjust h to a beam width equal to or greater than direction y. Accordingly, xcex8 and d are determined because of these limitations.
Next, the degree of the reduction of the interference noise as shown in FIG. 9 is calculated. Here, a case where a reflected last beam escapes without the effective diameter, and the energy is lost is considered. As indicated in FIG. 9, when a reflected beam that has reflected last is defined as the nth beam, an energy Rn of said nth beam is indicated by the following formula using reflection ratios r and r0.
Rn=rxc2x7(rxc2x7r0)nxc2x71xe2x80x83xe2x80x83(3)
In this case, the energy of the incident beam is defined as 1. When formula (3) is solved in relation to n, the following formula is obtained.
n=1n(Rn/r)/1n(rxc2x7r0)+1xe2x80x83xe2x80x83(4)
When r, r0, and Rn are given to formula (4), a termination beam number for the last beam n is determined. When n is defined, an effective diameter Y in the direction y is given by the following formula:
Y=(nxe2x88x921)xc2x7hxe2x80x83xe2x80x83(5)
Because the right side of formula (4) is a real number, n is determined as a value that count fractions as one said real number value.
Next, as for the degree of the reduction of the interference noise under a condition that the energy of a beam reflected at last at the nth stays within 1% of the incident energy, the cases when r0 is 99% (fixed) and when r is 80% and 96% are described. h is defined as 3 mm; the interference noise is assumed to be a speckle pattern.
First, as each intensity of n number of one-dimensional multi-beams is defined as Tk (k=1 to n) and as the distribution of speckle intensity in which each beam forms onto the reticule surface is defined as Ik (x,y) (k=1 to n), the distribution of the total intensity is given by the following formula:
I(x,y)=I1(x, y)+I2(x, y)+ . . . +In(x,y)xe2x80x83xe2x80x83(6)
The degree of the reduction of the interference noise is calculated by simulating statistic fluctuation a of I(x,y) inside the reticule surface in relation to average value  less than I greater than . When the calculation is made, it is assumed that the average value of Ik is proportioned to each beam intensity Tk; accordingly, an effect such that beam intensity Tk varies at each beam is incorporated. Tk is given by the following formula as the energy transmissivity of the partial reflection mirror is defined as t:
Tk=txc2x7(rxc2x7r0)(nxc2x71)kxe2x88x921
Since, in a general reference about the interference noise, C="sgr"/ less than I greater than  is called a speckle contrast and is defined as the degree of the interference noise, said definition is used for the simulation in this application. When n=1, speckle contrast C is standardized so as to be 100%. When contrast C is calculated, a transmissivity t of partial reflection mirror 202C is commonly presented at a denominator and a numerator; as a result, C does not depend on t.
Table 1 as shown below indicates n in relation to r of the partial reflection mirror, Y, and C by a computer simulation.
As is clear from Table 1, it is evident that the interference noise C is reduced by increasing r; however, it is also evident that effective the diameter Y in the direction y increases. This effect can be described with respect to the fact that, as r increases, the number of effective multiple beams n increases. More specifically, as n (the number of reflections) increases, the averaging of the interference noise easily occurs; and on the other hand, the effective diameter in the direction y increases according to formula (5).
As for a measure to reduce the effective diameter in the direction y, after the beam width has been reduced, beam distance h can be reduced. However, because the beam width and the beam divergence angle are generally inversely proportional to each other, even when the beam distance h is reduced by reducing the beam width at an incident location, the beam is expanded while the reflection is repeated; it is substantially difficult to reduce the effective diameter.
In addition, the reason that the loss of energy Rn of the last beam reflected the nth time occurs is because a nth reflection surface exists. For said reason, in order to reduce said loss, the nth reflection surface can be replaced with a anti-reflection coat (henceforth referred to as xe2x80x9can AR coatxe2x80x9d), or the reflection surface per se can be entirely removed. In this case, as r0, r, and Y are determined such that the intensities of an nth transmission beam Tn and a first transmission beam T1 are approximately equivalent, it is known from analysis that the interference noise can be efficiently reduced. However, in this case also, in order to further reduce the interference noise, r has to be increased, and the effective diameter in the returning direction has to be increased along with the increase of the number of reflections.
The present invention is produced in consideration of disadvantages as described above; the present invention aims to offer a compact and simple illumination device that can reduce ununiformity of the Gaussian intensity distribution due to a laser beam and an interference noise due to an optical integrator at the same time. Additionally, the present invention secondarily aims to offer a projection and exposure apparatus that uses said illumination apparatus and an exposing method.
In order to eliminate said disadvantages, the present invention offers an illumination apparatus that illuminates a mask with a predetermined pattern formed, characterized by comprising the following components: a light source that supplies a beam; a multi-beam generating optical system that consists of a reflecting member and a light splitting member, and converts the beam from said light source into a group of multiple beams; an optical integrator that splits the light from said light source and forms multiple light source images; wherein in the multi-beam generating optical system, said light splitting member is provided at a predetermined angle with respect to the reflecting member, and forms a plurality of beams by repeatedly reflecting the beam from said light source between said reflecting member and said light splitting member.
In a preferred embodiment of the present invention, during said multiple reflections, said predetermined angle is preferably adjusted so as to return the reflection light that has been reflected for a predetermined number of times returns in a direction of incidence.
In another preferred embodiment of the present invention, said multi-beam generating optical system is preferably structured such that among said group of multiple beams, the difference in length of two beams adjacent to each other is adjusted to be equivalent to the coherence length or longer, which is determined by a time coherence of said light source.
In another preferred embodiment of the present invention, a condenser optical system that guides the light passing through said optical integrator to said mask is preferably provided.
In another preferred embodiment of the present invention, at the incident surface of said optical integrator, a dispersion section that disperses each beam such that the diameter becomes larger than that of an element lens of said optical integrator, is preferably further provided.
In another preferred embodiment of the present invention, a relay lens system is preferably further provided between said dispersion section and said optical integrator.
The present invention also offers a projection and exposure apparatus characterized by comprising the following components: a first stage that holds said mask; an illumination apparatus that illuminates said mask as mentioned in any description above; a second stage that holds a substrate to be exposed; and a projection optical system that projects and exposes a pattern image of the mask, which is illuminated by using said illumination apparatus, on the substrate to be exposed.
In another preferred embodiment of the present invention, said projection and exposure apparatus is a scanning projection and exposure apparatus that makes a relative movement, and the scanning direction on said substrate to be exposed and the direction in which said group of multiple beams are arranged are preferably approximately equivalent.
The present invention also offers an exposing method that uses an illumination apparatus as mentioned in any description above, characterized by comprising the following steps: a step of illumination that illuminates said mask provided onto the object surface; and a step of projection that projects a pattern image of said mask onto a photosensitive substrate provided onto a final image surface.